Advanced combustion strategies for gasoline engines in the future depend on the existence of accurate pressure feedback from each combustion cylinder during the entire cycle (compression-combustion-exhaust cycle). These strategies may or may not include Homogeneous Charge Compression Ignition (HCCI) combustion, and can result in high pressure release rates which require fast and accurate pressure response. Placement of a sensor inside the combustion chamber requires drilling an extra hole in the engine's head. By using one hole for two functions, extra costs and other practical issues faced when drilling an extra hole in the engine's head can be avoided.
A pressure-measuring plug provided with an additional function is known from EP1790964A1. The plug comprises a ring-shaped sensing structure with an outer section and an inner core. A proximal end of the plug body is attached to the outer section. The inner core comprises a through hole for receiving a rod-shaped element. The ring-shaped sensing structure allows the inner core to move relatively to the outer section along a cylinder axis of the ring-shaped sensing structure when a pressure force is acting on the rod-shaped element. However, the pressure-measuring plug is not used for mounting in a cylinder head of a combustion engine.
EP2138819A1 and EP2444786A1 disclose a piezo-resistive pressure-measuring plug for a combustion engine for measuring a pressure. The piezo-resistive pressure-measuring plug comprises a plug body for insertion into a cylinder of the combustion engine, a rod that is arranged in the plug body, a sensing structure comprising a piezo-resistive element. The sensing structure is arranged between the rod and the plug body in such a way that the rod transmits the pressure in the combustion chamber of the cylinder to the sensing structure due to the pressure in the combustion chamber. A pressure change leads to an axial motion of the rod relative to the plug body so as to apply a force to the sensing structure. As the sensing structure is integrated in the plug body, the dimension of the hole in the sensing structure is limited.
Until now, spark plugs incorporating a pressure sensor have suffered from hysteresis, lack of sealing and sensitivity to external influences such as temperature.